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LIGHT MINERAL OILS  (BENZINE)                    341

When a more thorough separation of the different fractions is desired,
a flask surmounted by a dephlegmator may be used.

With very light, rectified oils, it is useful to evaporate a portion on the
water-bath to ascertain if any residue remains, or to allow a little to evaporate
spontaneously on a filter-paper to see if any oily spot is left.

Where exact results are required, allowance must be made for the atmospheric
pressure when this varies by as much as +_ 5 mm. from the normal (760 mm.).

3.  Flash Point.—For this purpose use is made of the inner vessel A
of the Abel apparatus (see Lamp Oil, Physical Tests, 4), this being placed
in a metal vessel about 6 cm. high and 9 cm. in diameter containing alcohol;
this vessel, in its turn, is placed in a larger metal vessel, also containing
alcohol and surrounded with felt.    Solid carbon dioxide is introduced into
the alcohol until the temperature in the liquid to be examined reaches
— 50° or —60°;   the temperature is then allowed to rise slowly and the
observations begun as with lamp oil.

This determination is seldom made, as it is known that naphtha has a low"
flash point, which is generally far below o°, although with some of the heavier
types it may be slightly above o°.

4.  Temperature of Ignition.—After the flash point has been deter-
mined, the cover of the vessel A is removed and for every 0-5° rise of tem-
perature a flame is brought near to the surface of the liquid.   The tem-
perature of ignition is taken as that at which persistent combustion of the
liquid itself takes place.

This determination also is rarely made for the reasons indicated above,
the temperature of ignition being only a few degrees (3 or 4) above the flash
point.

5.  Degree of Refining.—The extent of rectification of a light oil is
indicated by the following tests:

(a)  The oil is shaken with an equal volume of cone, sulphuric acid to
ascertain if the latter appears coloured after separation.

(b)  The oil is shaken 'with boiling water and the latter subsequently
tested with litmus and with barium chloride.

(c)   The liquid is boiled for a few minutes with alcohol containing a
few drops of ammonia and then treated with silver nitrate to see if any
brown coloration develops.

6.  Detection and Determination of Benzoles.

(i) DETECTION, (a) According to Holde, tar-pitch, previously washed
with petroleum benzine (D 070-071) until the latter dissolves no more,
is shaken with the light oil under examination. The latter becomes yellow
or brown if tar benzoles are present.

Not less than 5-10% of benzoles are detectable in this way.

(b) 5 c.c. of the oil, in a flask fitted with a reflux condenser and
immersed in a beaker of water, are treated with small quantities of fuming
nitric acid until evolution of red vapours ceases (20 c.c. of acid usually
suffice). At the end of the reaction, the contents of the flask are poured
into a graduated 100 c.c. cylinder containing 60% alcohol. The flask is